There are various types of viewing systems allowing a synthetic image to be superposed on an exterior environment. One possible solution, illustrated in FIG. 1, consists in employing a projector of stereoscopic images. The viewing system 10 then comprises:                a projector 11 of what are called “3D” stereoscopic images, said projector being capable of generating at least two images referred to as “right eye”/“left eye” images, representative of a given object. In the case in FIG. 1, the object is a sphere S;        a scattering semitransparent screen 12 onto which the “right eye”/“left eye” images are projected;        a pair of glasses 13 comprising means for separating the “right eye”/“left eye” images and first detecting means 14 and intended to be worn by a user;        second detecting means 15 associated with a fixed coordinate system R and that, in combination with the first detecting means 14, allow the spatial position of the pair of glasses 13 to be detected in this fixed coordinate system; and        an electronic processor 16 performing at least the following functions:                    acquisition of signals issued from the detecting means 14 and/or 15 and calculation of the position of the pair of glasses;            calculation of the position of the stereoscopic image corresponding to the position of the pair of glasses; and            calculation of the two right eye/left eye images.                        
There are various means for achieving the stereoscopic separation of the projected images.
In a first technical solution, temporal separation is used. The projector transmits sequentially and synchronously first the right eye image then the left eye image. The glasses are active and comprise active shutters that are synchronized with the projector. Thus, each eye perceives the image that is intended therefor and only said image. The shutters are generally produced in a technology based on liquid crystals. However, this solution has a number of drawbacks.
The active glasses require a power supply and control electronics and this raises maintenance issues in the context of use on board a vehicle. In addition, the polarizers of the LCD shutters obscure and may even completely obstruct the user's view of the cockpit, depending on the various polarization directions and the inclination of the glasses. Lastly, the presence of polarizers and the left-right vision alternation required to achieve the stereoscopic vision effect leads to loss of a substantial amount of light. The transmission of the glasses is thus no higher than 30%, resulting in a completely unacceptable darkening of the exterior landscape.
In a second technical solution, the stereoscopic projector functions in a polarized mode. It emits in succession and periodically a right eye image with a first polarization and a left eye image with a second polarization that is different from the first polarization. The pair of glasses 13 is passive. It comprises a first polarized eyeglass transparent at the first polarization and opaque at the second and a second polarized eyeglass transparent to the second polarization and opaque at the first.
The polarizer glasses are passive and solve the underlying problem of alternate occultation of each eye, and the management of batteries. In contrast, it is essential to use a silvered polarization-preserving projection screen. Since this screen is not transparent, it is not suitable for the applications concerned by the invention.
In a third technical solution, the projector emits two coloured images the emission spectra of which are separate. The pair of glasses comprises two different filters, the first transmits the first spectrum and filters the second spectrum. The second filter carries out the inverse function. Thus, each eye perceives one and only one coloured image and only said image. This technique is known by the name “anaglyph”. The simplest way of producing an anaglyph is to separate the visible spectrum into two portions, one red and the other blue. The obvious advantage of this device is how simple it is to implement, but vision of the exterior world is substantially altered.
Better still, the system referred to as spectral multiplexing separates the visible spectrum into two interlaced portions, one dedicated to each eye. However, although the perceived colour of the landscape is better preserved, its luminance is considerably decreased. The patent applications of Dolby Laboratories Licensing Corporation US 2011/0205494, US 2013/0342904 and US 2014/0022637 describe solutions of this type for cinematographic applications that require neither high light levels nor, of course, an exterior landscape to be transmitted.